A major unresolved problem in human organ transplantation is life threatening infection secondary to immunologically non-specific suppression of the immune response. The investigation supported by this grant concerns immunologically specific suppression of renal allograft rejection in the rat and rabbit as models which are potentially applicable to human problems. Recipients are treated with F(ab')2 or IgG from antidonor alloimmune sera. Cellular immunity and serum blocking factors are evaluated by three in vitro assays: 51Cr release, microcytotoxicity and mixed leukocyte culture.